Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. Such EUV mirrors have a mirror substrate and a reflection layer stack—constructed from a multiplicity of layer packets—for reflecting the electromagnetic radiation impinging on the optical effective surface. As mirror substrate material—for instance in the illumination device—metallic materials such as e.g. copper or aluminum or else—for instance in the projection lens—amorphous mirror substrate materials such as titanium dioxide (TiO2)-doped quartz glass (as sold under the trademarks ULE or Zerodur, for instance) are known.
Since adequate polishing of diverse (in particular metallic) mirror substrate materials cannot readily be achieved in terms of production engineering during the production of the mirror, in general use is made of additional polishing layers, e.g. composed of amorphous silicon (=a-Si), which can be processed with higher precision. However, here in practice the problem occurs, inter alia, that such polishing layers and possibly also the mirror substrate material itself, on account of the radiation loading resulting from the impinging EUV light, exhibit structural alterations, e.g. on account of compaction effects, which in turn affect the geometry of the applied reflection layer stack and thus the reflection properties of the mirror.
A further problem that occurs on account of the radiation loading resulting from EUV light during the operation of the projection exposure apparatus results from radiation-governed aging effects of the mirror substrate material itself, particularly if the amorphous mirror substrate materials mentioned above are used e.g. in the projection lens. In order to protect such mirror substrate materials and possibly also the polishing layers mentioned above, inter alia the use of protection layers (for short: SPL=“Substrate Protection Layer”) has proved to be expedient, which can be produced from a material that absorbs the EUV light to a comparatively great extent.
With regard to the prior art, merely by way of example, reference is made to the publications A. Ionascut-Nedelcescu et al.: “Radiation Hardness of Gallium Nitride”, IEEE Transactions on Nuclear Science Vol. 49 (2002), pages 2733-2738; Xueping Xu et al.: “Fabrication of GaN wafers for electronic and optoelectronic devices”, Optical Materials 23 (2003), pages 1-5 and P. J. Sellin et al.: “New materials for radiation hard semiconductor detectors”, CERN-OPEN-2005-005, pages 1-24.